Tailored profile pedestal for thermo-elastically stable cooling or heating of substrates

ABSTRACT

A shaped pedestal having a substantially convex profile for cooling a substrate or a substantially concave profile for heating a substrate. The tailored pedestal is formed of at least one continuous curvature segment that is generally elliptical, parabolic, or spherical in shape. The pedestal may be formed from more than one piecewise linear segments that are angled adjacent one another or stepped in a substantially convex or concave manner.

BACKGROUND OF THE INVENTION

1. Field of the Invention The present invention relates to devices for heating or cooling substrates, such as silicon wafers. Specifically, the present invention relates to a tailored pedestal for heating or cooling a wafer during processing.

2. Description of Related Art

Current practices for heating and cooling wafers using a conductive media, such as gas, utilize a constant gap between the heating or cooling device and the substrate. FIG. 1 depicts a substrate 10 shown over a flat heating or cooling device 12 initially with a constant gap 14 between them. Ideally, the constant gap leads to uniform heat transfer to or from the substrate. However, if a perturbation is added, such as an initial deformed shape of the wafer surface, an unstable thermo-elastic response may occur from the non-uniform heat transfer between the device and the substrate due to the gap variation. Initial distortion in the wafer may arise from film stress or from thermal gradients through the substrate thickness.

In the case of wafer cooling, an initial perturbation referred to as “doming” places the edge of the substrate closer to a cooling device than the center portion, as shown in FIG. 2. In FIG. 2, substrate 20 is shown “domed” or convex in shape with its center point 22 further from the flat surface of a cooling device 24 than the substrate's edge points 26. This shape will tend to cause faster cooling of the perimeter of the substrate from its initial shape 29, which in turn causes faster thermal contraction of the perimeter further leading to increase doming. FIG. 2 depicts this deformation increase 28 in the substrate surface due to the faster edge cooling. The increase in perimeter cooling relative to the center leads to increased doming or an unstable doming response.

In the case of wafer heating, an initial perturbation referred to as “cupping” places the center of the substrate closer to the heating device than its edges. FIG. 3 depicts an initially deformed substrate 30 in a “cupping” formation when exposed to a heating device 31. The cupping formation causes faster heating of the center of the substrate 32, which leads to faster thermal expansion of the center of the substrate and further cupping. The increased deformation is depicted by the lower substrate surface 34. This process is observable by pronounced “cupping” of substrates on heating devices.

In both instances, these formations are unstable, runaway conditions that lead to large thermal distortion of the substrate and highly non-uniform heat transfer. Of particular concern is the outer edge of the substrate transitions to tensile circumferential stress. If the substrate has edge defects, such as chips or cracks, the tensile stress condition may promote crack propagation and subsequent substrate breakage.

The present invention solves the problem of unstable wafer distortion during wafer heating or cooling. It also generates a biased stress state in the wafer that minimizes potential wafer breakage initiating from edge defects during heating or cooling.

SUMMARY OF THE INVENTION

Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a tailored profile pedestal to eliminate unstable wafer distortion during wafer heating or cooling.

It is another object of the present invention to provide a tailored profile pedestal to minimize potential wafer breakage initiating from edge defects.

A further object of the invention is to provide a device for thermo-elastically stable wafer cooling and wafer heating.

A further object of this invention is to provide a device for repeatable and uniform wafer cooling.

Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.

The above and other objects, which will be apparent to those skilled in art, are achieved in the present invention, which is directed to an apparatus for cooling a substrate comprising a thermally conductive, substantially convex shaped pedestal having a top surface, a center, and an edge, the pedestal top surface being in thermal contact with a lower surface of the substrate such that for a substantially flat substrate the pedestal center is initially located a closer distance to the substrate lower surface than the pedestal edge, and upon cooling activation, heat energy is transferred from the substrate to the pedestal substantially uniformly, cooling the substrate in a thermo-elastic stable manner. The pedestal top surface may comprise at least one continuous curvature segment, substantially convex in shape. The at least one continuous curvature segment may comprise a portion of an elliptical, parabolic, or spherical shape. The pedestal top surface may further include a substantially convex shape formed of more than one piecewise linear segments. The piecewise linear segments are angled relative to one another or stepped relative to one another to form the substantially convex shape. Upon the cooling activation the substantially convex shape of the pedestal top surface leads to a higher rate of cooling of a portion of the substrate lower surface closer to the pedestal center, and simultaneously to a lower rate of cooling of a portion of the substrate lower surface closer to the pedestal edge, promoting the thermo-elastic stability and compressive circumferential stress on the substrate. The thermal contact includes thermally conductive gas introduced between the pedestal top surface and the substrate lower surface. The substantially convex shape pedestal's depth of profile is determined by a gap distance between the pedestal top surface and the substrate lower surface.

In a second aspect, the present invention is directed to an apparatus for heating a substrate comprising a thermally conductive, substantially concave shaped pedestal having a top surface, a center, and an edge, the pedestal top surface being in thermal contact with a lower surface of the substrate such that for a substantially flat substrate the pedestal center is initially located a farther distance to the substrate lower surface than the pedestal edge, and upon heating activation, heat energy is transferred from the pedestal to the substrate substantially uniformly, heating the substrate in a thermo-elastic stable manner.

In a third aspect, the present invention is directed to a method for cooling a substrate having a lower surface comprising: introducing a pedestal having a center, an edge, and a substantially convex shaped top surface underneath the substrate lower surface such that for a substantially flat substrate the pedestal center is initially located a closer distance to the substrate lower surface than the pedestal edge; inserting a thermally conductive gas between the substrate lower surface and the pedestal top surface; and initiating a temperature differential between the substrate lower surface and the pedestal top surface wherein the pedestal top surface is at a lower temperature gradient than the substrate lower surface, such that heat energy is transferred from the substrate to the pedestal substantially uniformly, cooling the substrate in a thermo-elastic stable manner. The step of introducing the pedestal having the substantially convex shape top surface may comprise having the top surface formed of at least one continuous curvature segment, substantially convex in shape.

In a fourth aspect, the present invention is directed to a method for heating a substrate having a lower surface comprising: introducing a pedestal having a center, an edge, and a substantially concave shaped top surface underneath the substrate lower surface such that for a substantially flat substrate the pedestal center is initially located a further distance to the substrate lower surface than the pedestal edge; inserting a thermally conductive gas between the substrate lower surface and the pedestal top surface; and initiating a temperature differential between the substrate lower surface and the pedestal top surface wherein the pedestal top surface is at a higher temperature gradient than the substrate lower surface, such that heat energy is transferred from the pedestal to the substrate substantially uniformly, heating the substrate in a thermo-elastic stable manner.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the invention believed to be novel and the elements characteristic of the invention are set forth with particularity in the appended claims. The figures are for illustration purposes only and are not drawn to scale. The invention itself, however, both as to organization and method of operation, may best be understood by reference to the detailed description which follows taken in conjunction with the accompanying drawings in which:

FIG. 1 depicts a substrate over a heating/cooling device with a constant gap there between.

FIG. 2 depicts a deformation increase in the substrate surface due to faster edge cooling.

FIG. 3 depicts an initially deformed substrate in a cupping formation when exposed to a heating device.

FIG. 4 depicts the gap between a convex-shaped pedestal cooling device of the present invention and a substrate.

FIG. 5 depicts a concave-shaped pedestal heating device of the present invention.

FIG. 6 depicts cross-sectional views of convex-shaped pedestal cooling devices using continuous curvature segments and piecewise angular linear segments.

FIG. 7A depicts a theoretical wafer distortion during cooling under a flat pedestal with helium gas at atmosphere for a 300 mm silicon wafer.

FIG. 7B depicts a theoretical wafer distortion during cooling under a spherical pedestal with helium gas at atmosphere for a 300 mm silicon wafer.

FIG. 7C depicts empirical results of wafer distortion during cooling under a flat pedestal with helium gas at atmosphere for a 300 mm silicon wafer.

FIG. 7D depicts empirical results of wafer distortion during cooling under a spherical pedestal with helium gas at atmosphere for a 300 mm silicon wafer.

FIG. 8A depicts theoretical edge tangential stress during cooling under a flat pedestal with helium gas at atmosphere for a 300 mm silicon wafer.

FIG. 8B depicts theoretical edge tangential stress during cooling under a spherical pedestal with helium gas at atmosphere for a 300 mm silicon wafer.

FIG. 9A depicts the theoretical edge, center, and wafer average temperature of a flat cooling pedestal for an initially 0.5 mm domed wafer.

FIG. 9B depicts the theoretical edge, center, and wafer average temperature of a convex, generally spherically shaped cooling pedestal for an initially 0.5 mm domed wafer.

FIG. 10 depicts the theoretical temperature evolution for wafer cooling, comparing an initially flat pedestal and flat wafer combination to an initially spherical convex pedestal combined with a flat wafer.

FIG. 11A depicts the theoretical temperature evolution for wafer cooling, comparing an initially flat pedestal and initially cupped wafer combination to an initially spherical convex pedestal combined with an initially cupped wafer.

FIG. 11B shows the empirical results for typical wafer cooling evolution for comparing an initially flat pedestal to wafers having an initial 0.25 mm to 0.30 mm dome shape and an initially spherical convex pedestal combined with an initially cupped wafer.

FIG. 12 is a table comparing the cooling results for a flat pedestal design versus a convex shaped cooling pedestal design.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

In describing the preferred embodiment of the present invention, reference will be made herein to FIGS. 1-12 of the drawings in which like numerals refer to like features of the invention.

Two embodiments are presented to eliminate unstable wafer distortion during wafer heating or cooling, minimize wafer cracking, and promote thermo-elastically stable wafer cooling and heating. One embodiment is directed to domed wafers during wafer cooling, and the other is directed to cupped wafers during wafer heating. The embodiments modify the surface of the cooling and heating devices, and benefit from the geometric differences between the cooling or heating device surface and the adjacent wafer surface.

FIG. 4 depicts the gap 44 between a convex-shaped cooling device 40 and a flat substrate 42. The gap 44 is shown increasing from the substrate center 46 to the substrate edges 48 by virtue of the shaped cooling device surface. Combined with a gas introduced as a conductive media, the shape of the gap leads to a higher rate of cooling at the center 46 of the substrate, and a lower rate of cooling towards the edge 48 of the substrate. The center 46 contracts faster than the edge from the thermal elastic effect, causing a stable condition in which the wafer is prone to remain flat. Since the outer perimeter of the substrate is pulled inward by the faster contracting center, the outer region of the substrate, including the edge, transitions to a state of compressive circumferential stress. This condition tends to compress wafer cracks or other edge defects rather than pull them apart. The compressive stress state suppresses any crack propagation and minimizes the potential for the wafer to break during cooling.

FIG. 5 depicts a concave-shaped heating device 50 of the present invention. The gap 52 between the heating device 50 and a flat substrate 54 decreases from the center 56 of the substrate to the substrate edge 58. Combined with a conductive media, such as the thermally conductive gas, the concave-shaped heating device 50 through proximity to the bottom surface 53 of the substrate 54 leads to a lower rate of heating at the center 56 of the substrate, and a higher rate of heating at the substrate edges 58. In this design, the perimeter of the substrate expands more quickly than the center from thermal expansion effects, causing a stable condition in which the substrate remains substantially flat. Since the outer perimeter of the substrate is pulled inward by the more slowly expanding center, the outer region of the substrate, including the edge, transitions to a state of compressive circumferential stress. If there are edge defects in the substrate, such as chips or cracks, the compressive stress state suppresses crack propagation and minimizes the potential for wafer breakage during heating.

In both heating and cooling conditions, the preferred profile of the shaped pedestal is such that the cooling device is generally convex and the heating device is generally concave. Different shapes will initiate different stress profiles in the substrate. The depth of the profile is determined by the desired total variation in the gap. These shapes include, but are not limited to, the cross-sectional examples depicted in FIG. 6. The tailored pedestal exhibits either a generally convex shape for cooling or a generally concave shape for heating, independent of whether the pedestal is formed from piecewise linear segments or by a continuous curvature. Four convex shapes are presented in FIG. 6 for cooling, although other segmented variations are not excluded and would be too numerous to illustrate. Additionally, these surface topographies may be reversed and used as concave surfaces for heating. An approximate spherical shape 62 is shown as one type of pedestal design. Other curved shapes may represent this approximate spherical shape, such as by small segments of a large ellipse or parabola. This approximate spherically-shape pedestal 62 is shown as a continuous curvature. An approximate spherically-shape pedestal allows the substrate to be thermo-elastically stable for initial distortions in the same direction, and less than the depth of the profile. For example, a 0.5 mm depth of profile on a cooling device would be stable for an initial doming distortion on the substrate up to 0.5 mm. Substrates will always be stable for any initial distortion in the direction opposite of the device profile. Other suggested profile shapes include angled linear segments, such as a step-function shape 63, a two-zone taper 64, having a first tapered zone 65 and a more angled second tapered zone 66, and an n-zone taper 67. The linear segments may also be combined with slightly curved segments while sustaining the generally convex or concave shapes.

Empirical results of wafer distortion and theoretical results of tangential wafer edge “hoop” stress time histories were analyzed during cooling with helium gas at atmosphere for a 300 mm silicon wafer. Comparisons were made between a standard flat pedestal cooling device and a tailored convex shaped cooling device. In the case of the flat cooling device, the gap between the substrate and the pedestal was held at a constant 0.035 inches, and in the case of the tailored cooling device, the gap varied from 0.025 inches at the center and 0.045 inches at the edge, which is the radius length of 150 mm from the center in the spherical profile. The dimensions are such that the average gap is the same for both cooling devices. On the flat cooling device, wafers with initial domed shapes demonstrated increased distortion during cooling and were found to develop tensile edge hoop stress during cooling. However, with respect to the tailored cooling pedestal, wafers with initial domed shapes of up to 0.5 mm demonstrated decreasing or constant distortion and compressive edge hoop stress. Initially cupped wafers were stable on both devices. The cupped wafers were predicted to be more stable during cooling, showing decreasing distortion and compressive edge hoop stress. These empirical and theoretical results support the preferred design shape being more generally convex for cooling pedestals. Conversely, a domed wafer is predictably more stable during heating than a cupped wafer, supporting a preferred concave heating pedestal.

Empirical results of wafer distortion during cooling with helium gas at atmosphere is depicted in FIGS. 7C and 7D for a 300 mm silicon wafer. The figures represent a comparison between a standard flat cooling device and a tailored convex cooling device (spherical pedestal) for initially domed and initially flat wafers. For example, as depicted in FIG. 7A, line A represents the typical theoretical distortion of a wafer with an initial dome of 0.25 mm to 0.3 mm, showing a peak distortion of greater than 2 mm after six seconds of exposure. In comparison, when a spherical pedestal is used, the initial theoretical distortion is reduced to near zero during cooling, as shown in FIG. 7B. The empirical results of FIGS. 7C and 7D confirm these calculations.

FIGS. 8A and 8B depict a comparison of theoretical tangential wafer edge stress between a flat cooling device and a tailored convex cooling device for various wafer initial conditions. As depicted in FIG. 8A, line C represents peak edge hoop stress in MPa for a 0.5 mm domed wafer when cooled under a flat pedestal. A peak tensile stress of over 7 MPa is predicted. In comparison, line D depicts the same wafer's edge hoop stress in the ideal case, when cooled by a spherically shaped pedestal. No tangential edge hoop stress is predicted. In all cases the tangential edge stress is predicted to be zero or compressive up to −7 MPa with the domed pedestal.

The edge, center, and wafer average temperatures were considered when comparing a flat cooling pedestal with a convex spherically-shaped cooling pedestal for an initially 0.5 mm domed wafer as shown in FIG. 9. An 80° C. difference was demonstrated between the center and the edge of the wafer when subject to the flat cooling pedestal after approximately 11 seconds of exposure, which is a result of large induced distortions. This wafer distortion leads to highly non-uniform and inefficient cooling. The average wafer temperature at the end of 11 seconds of cooling was predicted to be 96° C. However, when using a convex, generally spherically shaped cooling device of the present invention, the 0.5 mm domed wafer represented an ideal case study as the pedestal was made to exactly match the cooling pedestal shape yielding a constant gap distance between the pedestal and the substrate. As shown in line E of FIG. 9B, the wafer cooling is ideally uniform and the average temperature after eleven seconds is significantly lower, 84° C. Because the distortion is controlled or reduced for the tailored pedestal, the cooling gap is more consistently maintained, and wafers may actually be cooled faster with wafer stress bias to resist wafer breakage. Empirical results for a wafer with an initial dome of 0.25 to 0.3 mm are shown in FIG. 11B show a result consistent with the theoretical prediction.

Although these predictions considered convex shaped cooling pedestals with flat or domed wafers, the converse also follows suit. Concave shaped heating pedestals have similarly expected results when comparisons are made with flat and cupped shaped wafers. The cupped wafers being unstable under heating conditions, as are the domed wafers under cooling conditions.

Additionally, a shaped pedestal will not contribute detrimentally to the cooling or heating of a flat wafer. FIG. 10 depicts the temperature evolution for wafer cooling, comparing an initially flat pedestal and flat wafer combination to an initially spherical convex pedestal combined with a flat wafer. FIG. 11A depicts the theoretical temperature evolution for wafer cooling, comparing an initially flat pedestal and initially cupped wafer combination to an initially spherical convex pedestal combined with an initially cupped wafer. Good performance is exhibited in both instances for both cooling devices because in all cases the thermo-elastic response is stable. The convex pedestal will not adversely affect wafers that are flat or cupped, while greatly enhancing the cooling performance of domed wafers. In the alternative, a concave pedestal will not adversely affect wafers that are flat or domed, while greatly enhancing the heating performance of cupped wafers. FIG. 11B shows the empirical results for typical wafer cooling evolution for comparing an initially flat pedestal to wafers having an initial 0.25 mm to 0.30 mm dome shape and an initially spherical convex pedestal combined with an initially cupped wafer.

The analytical results are summarized in the table shown in FIG. 12. For the extremes of initial wafer shape, the average wafer temperature at the end of eleven seconds ranged from 33° C. to 96° C. for a flat pedestal, and from 55° C. to 84° C. for a convex cooling device, showing the convex device controlling a tighter range. The maximum center-to-edge gradient in the wafers during cooling ranged from −29° C. to 80° C. for the flat pedestal, and −46° C. to 0° C. for the shaped pedestal, showing that cooling uniformity is better when using a convex pedestal for the extremes of initial wafer shape. An initially domed wafer exhibits increased distortion during cooling for a flat pedestal, but stable shape when cooled on a shaped pedestal. Also, the peak edge hoop stress ranges from −550 psi (compressive) to 1050 psi (tensile) when using a flat pedestal to −1000 psi (compressive) to 0 psi (tensile) when using a shaped pedestal. The negative psi represent a compressive force on edge defects and cracks, whereas a positive tensile force tends to pull edge defects and cracks apart.

The present invention teaches tailored pedestals for heating and cooling wafers or substrates during processing, wherein the shape of the tailored pedestal is generally convex for cooling conditions and generally concave for heating conditions.

While the present invention has been particularly described, in conjunction with a specific preferred embodiment, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art in light of the foregoing description. It is therefore contemplated that the appended claims will embrace any such alternatives, modifications and variations as falling within the true scope and spirit of the present invention. Thus, having described the invention, what is claimed is: 

1-18. (canceled)
 19. A method for cooling a substrate having a lower surface, a center area, and an edge area, the method comprising: introducing a pedestal having a center, an edge, and a substantially convex shaped top surface underneath said substrate lower surface such that for a substantially flat substrate said pedestal center is initially located at a closer distance to said substrate lower surface than said pedestal edge; inserting a thermally conductive gas between said substrate lower surface and said pedestal top surface through the gap between the pedestal edge and the substrate lower surface at such process conditions that the thermally conductive gas initiates heat transfer from; and said substrate lower surface to said pedestal top surface causing faster contraction of the center area than the edge area of the substrate causing substrate flattening during cooling.
 20. The method of claim 19, wherein said substantially convex shape top surface comprises AT least one continuous curvature segment, substantially convex in shape.
 21. The method of claim 20 wherein said at least one continuous curvature segment comprises a portion of an elliptical, parabolic, or spherical shape.
 22. The method of claim 19, wherein said substantially convex shape top surface comprises more than one piecewise linear segments.
 23. The method of claim 22 wherein said piecewise linear segments are angled relative to one another to form said substantially convex shape.
 24. The method of claim 22 wherein said piecewise linear segments are stepped relative to one another to form said substantially convex shape.
 25. A method for heating a substrate having a lower surface, a center area, and an edge area, the method comprising: introducing a pedestal having a center, an edge, and a substantially concave shaped top surface underneath said substrate lower surface such that for a substantially flat substrate said pedestal center is initially located at a further distance to said substrate lower surface than said pedestal edge; inserting a thermally conductive gas between said substrate lower surface and said pedestal top surface through the gap between the pedestal edge and the substrate lower surface at such process conditions that the thermally conductive gas initiates heat transfer from; said pedestal top surface to said substrate lower surface causing slower expansion of the center area than the edge area of the substrate causing substrate flattening during heating.
 26. The method of claim 25, wherein said substantially concave shape top surface at least one continuous curvature segment, substantially concave in shape.
 27. The method of claim 26 wherein said at least one continuous curvature segment comprises a portion of an elliptical, parabolic, or spherical shape.
 28. The method of claim 25, wherein said substantially concave shape top surface comprises more than one piecewise linear segments.
 29. The method of claim 28 wherein said piecewise linear segments are angled relative to one another to form said substantially concave shape.
 30. The method of claim 28 wherein said piecewise linear segments are stepped relative to one another to form said substantially concave shape.
 31. The method of claim 19, wherein the substrate is flatter after cooling than before cooling.
 32. The method of claim 19, wherein the substrate comprises a 300-millimeter wafer distorting less than about 0.5 millimeters during cooling.
 33. The method of claim 32, wherein the 300-millimeter wafer has a dome deviation of at least about 0.1 millimeter before cooling.
 34. The method of claim 19, wherein the substrate comprises a 300-millimeter wafer and wherein said substrate lower surface is initially located closed to said pedestal center than to said pedestal edge by about 0.020 inches.
 35. The method of claim 19, wherein the temperature of the substrate center area is within less than about 50° C. from the temperature of the substrate edge area during cooling. 